Solid-liquid separation methods are important in a variety of industries, including, but not limited to, the chemical industry, the pharmaceutical industry, and the water and waste treatment industry. Such solid-liquid separation methods vary, and may include, but are not limited to, vacuum or pressure filtration, centrifugation, sedimentation and clarification. In many chemical processes, these solid-liquid separation methods often play a critical role in the manufacture of particular chemical intermediates. For instance, the purification of para-xylene for the manufacture of terephthalic acid has historically required centrifugation to achieve para-xylene purity levels of about 99.7%.
The purification of para-xylene typically begins with a C8 aromatic hydrocarbon feed that typically comprises ethylbenzene and mixture of xylene isomers, such as ortho-xylene, meta-xylene and para-xylene. Processes to separate these xylene isomers include low temperature crystallization, fractional distillation and adsorption.
Crystallization is often preferred for separating para-xylene from the C8 aromatic feedstream because while xylene isomers have undesirably similar boiling points, they have dramatically different melting points. Pure para-xylene freezes at 56° F., pure meta-xylene freezes at −54° F., pure ortho-xylene freezes at −13° F., and pure ethylbenzene freezes at −139° F.
The recovery and purification of para-xylene from a mixture of xylene isomers by crystallization are typically limited by the formation of one or the other of two binary eutectics, the para-xylene/meta-xylene binary eutectic or the para-xylene/ortho-xylene binary eutectic. Depending on the starting composition of the mixture, para-xylene will crystallize from the mixture as the temperature of the mixture is lowered, and the mother liquor composition will approach one of the binary eutectic compositions. If the temperature falls below either of the binary eutectic temperatures, then a second solid phase which is lean in para-xylene will crystallize from the mixture. The formation of a second solid phase is generally viewed as undesirable so crystallization processes are typically operated at a temperature warmer than the warmest binary eutectic temperature. While this limits the recovery of the process, conventional para-xylene separation processes that use crystallization produce a substantially pure para-xylene product.
For example, U.S. Pat. No. 3,177,265, which is incorporated herein by reference, illustrates a conventional indirect-cooled crystallization process for purifying para-xylene. In this process, a C8 aromatic feedstream comprising about 20 percent para-xylene with the remaining components ortho-xylene, meta-xylene, and ethylbenzene is crystallized in a series of crystallization stages to form a mixed xylene slurry while utilizing costly centrifugation steps to separate the slurry into a crystal cake and a liquid filtrate. This para-xylene purification process produces a para-xylene product with a purity in excess of 98 percent.
Although such processes produce a para-xylene product with a purity level in excess of 98 percent, the use of centrifuges add significant costs to the purification process due to their high capital costs and the high maintenance costs inherent in high speed rotating parts. As a result, prior efforts have focused on developing alternatives to centrifugation to improve the economics of producing substantially pure para-xylene.
Two such efforts are U.S. Pat. Nos. 4,734,102 and 4,735,781, to Thijssen which disclose an apparatus and process for concentrating a suspension. The Thijssen apparatus, called a hydraulic wash column, is directed to a hollow cylinder in which one or more tubes of a constant outer diameter extend in an axial direction within the wall of each tube comprising at least one filter being mounted forming the only direct connection between the interior of the tube and the interior of the hollow cylinder.
The Thijssen process separates solids from liquids by directing a suspension into a first end of the hydraulic wash column and a wash liquid into a second end of the hydraulic wash column in countercurrent flow to the suspension, forming a bed in the hollow cylinder. A filtrate (mother liquor) from the suspension escapes through the filters of the filter tubes into the interior of the tubes, and a concentrated suspension is withdrawn from the second end of the hydraulic wash column. A liquid is introduced at the second end to reslurry the concentrated suspension. This liquid also acts as the wash liquid. When the process is used to separate a suspension derived from a melt crystallization process, the wash liquid comprises molten crystal product from the suspension.
Although the Thijssen patents teach an alternative method and apparatus for solid-liquid separation, the Thijssen process cannot effectively separate liquids from solids at temperatures far below the melting point of crystals in slurries derived from a melt crystallization process because the wash liquid utilized during the process freezes within the Thijssen hydraulic wash column during the washing part of the operation. At lower and lower temperatures, the freezing wash liquid fills a larger portion of the void fraction between the solids thereby requiring higher and higher pressures to drive the wash liquid into the column. Eventually, a low enough temperature will be reached wherein the freezing wash liquid essentially plugs the device causing failure and imminent shutdown of the Thijssen apparatus and process.
Additionally, the use of a molten solids wash liquid in the Thijssen process can contaminate the filtrate with a liquid that cannot easily be separated from the filtrate and result in a substantial loss of solid product to the filtrate.
Consequently, there is still a need for alternative methods and apparatuses for solid-liquid separation that: (1) separate liquids from solids in slurries derived, for example, from a crystallization process without the unnecessary loss of solids to the filtrate during the separation process; (2) direct separated filtrates and/or product cakes for further processing without significant energy and/or cost penalties; and (3) operate cooperatively and in conjunction with conventional unit operations.
It has now been found that feeding a displacement fluid, such as a gas, in lieu of a wash liquid produces a relatively dry and pure product with sufficient solids content that can be further processed with little or no additional refrigeration costs.
It has also been found that separating liquids from solids in a filter column, as described herein, at temperatures far below the melting point of crystals in slurries derived from a crystallization process can be operated in a continuous manner without high loss of the crystals to the liquid filtrate through one or more filters during the separation process.
It has also been found that passing a substantial portion of a displacement fluid through a solid packed bed of crystals to one or more filters can result in an acceptably pure solid product.